Conventional LED dimming control methods mainly have two ways to control brightness: pulse width modulation (PWM) dimming and direct-current (DC) dimming. PWM dimming process switches a switch to control the average of LED current, and thus if use PWM dimming to control LED brightness, the LED brightness can be linearly controlled by LED average current IF_avg as shown by the curve 10 in FIG. 1. DC dimming process controls the LED current, and thus if use DC dimming to control LED brightness, the LED brightness is not linearly proportional to LED average current IF_avg as shown by the curve 12 in FIG. 1. It is difficult for DC dimming to achieve linear LED dimming control. However, although PWM dimming can achieve linear LED dimming control, in some brightness, for example, the level BR designated in FIG. 1, the LED average current IF_avg by DC dimming is less than by PWM dimming. Therefore, DC dimming can save input power to get a same brightness. On the other hand, as shown in FIG. 2, LED brightness is also affected by ambient temperature. For example, when the temperature of LED is lower, the brightness is higher. Therefore, it could decrease LED current to achieve proper brightness to save power.